Math Tools
Percentage Calculator for 5 Common Problems
Find a percentage of a number, one group's share of two groups, a value after an increase or decrease, and simple annual interest with the formula and calculation base made explicit.
Five formulas, one clear workspace
Choose the percentage problem
The labels and substituted formula change with the selected mode so the calculation base remains visible.
Formula with substituted values
Calculation result
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- Calculation base
- —
- Difference or interest
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- Rounding
- Half up, up to 6 decimals
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Quick answer
How do I calculate a percentage?
Percent means parts per hundred, so convert p% to p ÷ 100 before multiplying the base value. To find a share, divide the part by the total and multiply by 100. This calculator separates five problems so you can see exactly which number is used as the base.
How to use this tool
- Choose one of the five modes and read its short formula to confirm the calculation base.
- Enter the value, percentage rate, and years shown for that mode; the result updates automatically.
- Review the substituted formula and explanation before copying or exporting to Print, Excel, PDF, or TXT.
How the calculation works
Every decimal is converted to an integer scaled by 1,000,000 and calculated with BigInt to avoid binary floating-point drift. Results are rounded half up to at most 6 decimal places. Mode 1 uses value × rate ÷ 100; mode 2 uses first ÷ (first + second) × 100; increase and decrease use value × (1 ± rate ÷ 100); simple interest uses I = P × r × t without compounding.
Examples
| Example | Input | Expected result |
|---|---|---|
| 20% of 1,200 | |
240 because 1,200 × 20 ÷ 100 = 240 |
| 15 within two groups of 15 and 25 | |
37.5% because the combined total is 40 and 15 ÷ 40 × 100 = 37.5 |
| Increase 500 by 10% | |
550 |
| Simple interest | |
Total interest 300 and principal plus interest 5,300 |
Limitations
- Share mode divides the first value by the sum of both values, not by the second value, and cannot calculate when the combined total is zero.
- Decrease mode accepts 0–100% so the result cannot become negative; other modes accept non-negative rates up to 1,000,000%.
- Inputs must be non-negative, no more than 999,999,999,999, and have at most 6 decimals; results are rounded half up to at most 6 decimals.
- Interest is simple, not compound interest, APR, or an amortization schedule, and the result is not financial advice.
- Increase and decrease are relative to the starting value; the calculator does not compute percentage-point differences.
Frequently asked questions
What is the difference between percent and percentage points?
If a rate rises from 20% to 25%, the difference is 5 percentage points. Relative to the original 20%, it increased by 25%. State whether you mean the difference between rates or a relative change.
Why does the share mode add the first and second values?
This mode asks what percentage the first group represents of both groups combined, so it uses first ÷ (first + second). If you already know a total, enter the remainder as total minus the first group.
Will the interest result match a real loan or savings account?
Not necessarily. This mode uses only I = P × r × t simple interest. It does not model daily balances, compounding, fees, taxes, or contract terms.
Does the website store the numbers I enter?
No. Calculation, copying, and file generation happen in your browser without sending entered values to the server.
References
- BIPM — SI Brochure: percent means parts per hundred Verified 2026-07-31
- Consumer Financial Protection Bureau — Simple interest formula worksheet Verified 2026-07-31
- Legacy THcount — five percentage calculations Verified 2026-07-31
Content and calculation method last reviewed: